THE PLASTICITY OF SOLIDS

239

In conclusion, we note again, cf. page 58, that Reiger (1906)
and Glaser (1907) have carefully investigated the question as to
whether the laws of Poiseuille may be applied to soft solids, using
as their material suspensions of colophony in turpentine. They
concluded that with a tube having a radius of 0.49 cm the vis-
cosity was independent of the pressure between the limits of
136 and 2,172 g per square centimeter; and in a similar way it
was independent of the length of the tube for lengths varying
between 2.4 and 20.6 cm. They found that with a pressure of
1,965 g per square centimeter, if they varied the radius of the
tube from 1.52 to 0.34 cm, the viscosity remained constant but
for tubes of smaller radii the viscosity rapidly increased until
finally the material seemed to have infinite viscosity. This
inferior limit is unlike anything observed in the flow of liquids, for
the smaller the radius of the tube, the better are the laws of
Poiseuille obeyed, and in large tubes the flow is largely inde-
pendent of the viscosity of the fluid. It seems probable that the
use of such very large tubes has prevented Reiger and Glaser
from discovering the friction constant just as, in the period before
Poiseuille's study of flow in capillaries, the use of large tubes
prevented the discovery of the laws of viscous flow. In large
tubes the shearing stress is very large in comparison with the
friction which may possibly explain the fact that the "viscosity"
was found to be independent of the pressure or length of the tube.
We note that the inferior limit of the radius of the tube is
increased as the percentage of solid in the mixture is increased.
This is what we should expect since this procedure raises the
friction constant. With an 80 per cent of colophony the lower
limit of the radius was found to be 0.100 cm, with an 85 per cent
mixture it was 0.576 cm., and with a 90 per cent mixture it was
1.019 cm. We give below a r€sum6 of the data of Glaser for the
90 per cent suspension of colophony in turpentine, the pressure
throughout being 2,040 g. per square centimeter.
The subject of the plasticity of ice takes on exceptional
interest and importance in connection with the flow of glaciers
and it has been the object of research by many investigators,
among whom we mention Pfaff (1875), McConnel (1886),
Miigge (1895), Hess (1902), Weinberg (1905) and Deeley and
Parr (1914). It is a< noteworthy fact that the precipitous moun-